Recently, an organic light emitting display device has been paid attention to as a next-generation flat type display device. The organic light emitting display device has excellent properties such as a self-luminous property, a wide viewing angle property, and a high-speed response property.
The structures of prior art organic light emitting devices are configured to form a glass substrate on which a first electrode consisting of ITO or the like; an organic layer consisting of a hole transport layer, a light emitting layer, and an electron transport layer, or the like; and an upper electrode having a low work function, and the emitted light passes through the first electrode having a transparency and is taken out from the rear surface of the substrate.
However, if an active matrix which has the advantage of having a higher precision and larger image plane as compared to a simple matrix is used, the aperture ratio is limited in an organic light emitting display device which takes out emitted light from the rear surface of a substrate. Especially, in a large-screen display device, in order to reduce brightness fluctuation among pixels due to voltage dropping of a power line, it is required to widen the width of the power line, thereby resulting in an extremely small aperture ratio.
In the circumstances, there is an approach for taking out emitted light from the upper electrode side by making the upper electrode transparent. If the upper electrode is made to be transparent, the upper electrode is formed by a sputter film deposition process using an indium-oxide based oxide such as ITO or IZO.
As for such an organic light emitting element of top emission type, the following patent document 1 JP-A-2000-58265) discloses an organic light emitting element having an organic cathode buffer layer on an organic light emitting structure, as a protection layer against damage caused by depositing a high energy with the cathode.
In an organic light emitting element having a top emission type structure, it is required for a layer inserted between the upper electrode and an organic film to have a thin film thickness in order to avoid problems such as low transmission or low conductivity. Therefore, when the upper electrode is formed, it is not possible to prevent the organic film from being oxidized, thereby resulting in a problem that the light emission voltage increases.
According to the above-mentioned patent document 1, although the organic buffer layer can be protected from the damage during the high energy deposition, the buffer layer itself is oxidized during the formation of the upper electrode. As a result, the increase of the light emission voltage cannot be avoided.